why wouldn't it be? losing it is pretty much the standard procedure - only very, very few old(er) games have their source code available (be it public or non-public). for whatever reason, "release your source code to the public once it has no more commercial value so your game(s) would live forever" is not a very popular way of ending a games' lifecycle.
lets see whether I can dig up some examples of old games that actually do have their source available;
-[url=https://en.wikipedia.org/wiki/Doom_(1993_video_game)]Doom[/url] (public. id soft were one of the few who understood this from day one)
-[url=https://en.wikipedia.org/wiki/Marathon_(video_game)]Marathon[/url] (public)
-
Duke3D (public)
-
Shadow Warrior (public)
-[url=https://en.wikipedia.org/wiki/Blood_(video_game)]Blood[/url] (non-public, owner decided to not release)
-
Rise of the Triad (public)
-all
Dark games (the code exists, but current status unknown, and is still owned by two separate entities)
-[url=https://en.wikipedia.org/wiki/Thief:_Deadly_Shadows]Thief Deadly Shadows[/url] (the fairly recent xbox360 release suggests that the code still exists)
-Turok (NightDive apparently having the code, making the enhanced GOG release possible)
-most
Unreal1 powered games
those are ones that I remember, you can continue
here - but you can see right away that the list has maybe 200 entries? considering the thousands, if not tenths of thousands games released in the last quarter of the century, it's pretty much nothing. that's why fans of classic games go through the painstaking process of reverse engineering or recreating them from scratch - unless they are C&D'd by the very same idiots who have managed to lose the source code in the first place (many companies are completely nuts about this - they don't care about their games or the source code, but will relentlessly attack anyone who they feel is even loking at their IP in a wrong way).
you are perhaps confusing the SS1 source code with SS2 source code, which was known to be in EA's possession, and was also leaked a couple of years ago.